The present invention relates in general to identifying signal quality of a received AM radio transmission, and more specifically, to detecting the presence of an AM signal of sufficient reception quality to stop a scan tuning operation.
Receivers for commercial radio broadcasts are typically equipped with scan and/or seek functions. In response to the initiation of a scan or seek operation, a receiver examines consecutive allocated frequencies for a broadcast having sufficient reception quality. When such signal is detected, the scan or seek function is paused or terminated to allow audible reproduction of the broadcast.
Several different methods have been employed to generate a stop signal for terminating the frequency sweep operation in an automatically scanning tuner. Most methods rely on a measure of the received signal strength, a frequency window determined by a quadrature detector or an IF counter circuit, or a combination of these. Some methods also employ detection of the noise level associated with an allocated channel being examined. AM radio receivers especially rely on a measure of received signal strength in generating a stop signal.
Scan tuning operations in an AM receiver tend to be slow. As with substantially all radio receivers, an AM receiver is provided with automatic gain control (AGC). Automatic gain control is a well-known technique for maintaining a substantially constant level of an amplified output signal even though the input signal level is varying. In broadcast communication receivers, AGC is employed to provide a consistent audio output level despite fluctuations in signal strength of a tuned-in broadcast signal or fluctuations occurring while retuning to other broadcast signals.
In the case of an AM receiver, the AGC loop must have a long enough time constant to avoid filtering out the modulated audio information on the AM carrier; otherwise, the AGC loop would remove the AM modulation and destroy the audio information. Thus, in a scanning AM receiver, the decision time to determine whether a quality broadcast is being received based on a measure of signal strength must be longer than the AGC loop response time. This causes undesirable delays in finding an acceptable broadcast signal during a scan tuning operation.
Another problem in prior art scanning AM receivers relates to stopping on a broadcast signal having adjacent channel interference. The presence of a strong broadcast signal on an adjacent channel adds to the signal strength measure at the current weak channel of interest, thereby inflating the true signal strength at that frequency. Thus, a scan-tune function may stop at a broadcast signal having unacceptable reception quality.